What is the need for modulation?
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Answer Posted / meenal
the need of modulation first arose for radio transmission of low frequency(audio frequency-20Hz to 20KHz) signals.
few imp points are:
1)Ease of Radiation:
,
2)Simultaneous Transmission of Several Signals:
consider the case of several radio stations broadcasting audio baseband(msg) signal directly,without any modification.They would interfere with each other bcoz the spectra of all the signals occupy MORE or LESS the same Bandwidth.thus,it would be possible to broadcast from only one radio or television station at A TIME.
This is wasteful because the CHANNEL BANDWIDTH may be much larger than that of signal(to understand this statement u must know d diff. betn channel and signal BW)
ONE way to solve this problem is to use MODULATION...
we can use various audio signals to modulate different carrier freq.,thus TRANSLATING EACH signal to a different frequency range..
if the various carriers r chosen suficiently far apart in frequency,the spectra of modulating signal will not overlap.
and thus will not interfere with each other.
AT the receiver 1 can y=use a tunable band pass filter to select the DESIRED station or signal.
THIs method of TRANSMITTING SEVERAL SIGNALS SIMULTANEOUSLY IS KNOWN AS FREQUENCY DIVISION MULTIPLEXINg. in FDM the bandwidth of the Channel is shared by various mesage signals without any overlapping...
anothe one is TDM (time division mux);which is suitable when a signal is in the form of a pulse train (as in PCM)
here the pulses are made narrower,and the spaces left between pulses are utilized by pulses from other signals.
thus in effect,in TDM ,the TRANSMISSION TIME IS SHARED BY a no. of signals interleaving the pulse train of various signals in specified order..
now at receiver ,pulse trains corresponding to various signas get seperated n msg is recovered.
3)EFFECTING THE EXCHANGE OF SNR with BANDWIDTH..
THE AMOUNT OF MODULATION (modulation index)used controls the exchange of SNR(signal to noise ratio) and the transmission bandwidth..FM or PM can effect such exchange...
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